1. HOW TO MODEL GALACTIC CHEMICAL EVOLUTION

Before going into the detailed chemical evolution history of the Milky
Way and its satellites, it is necessary to understand how to model, in
general, galactic chemical evolution.
The basic ingredients to build a model of galactic chemical evolution
can be summarized as :

Initial conditions;

Stellar birthrate function (the rate at which
stars are formed from the gas and their mass spectrum);

Stellar yields (how elements are produced in
stars and restored into the interstellar medium);

Gas flows (infall, outflow, radial flow).

When all these ingredients are ready, we need to write a set of
equations describing the evolution of the gas and its chemical
abundances which include all of them.
These equations will describe the temporal variation of the gas content
and its abundances by mass (see next sections). The chemical abundance
of a generic chemical species i is defined as:

(1)

According to this definition it holds:

(2)

where n represents the total number of chemical species.
Generally, in theoretical studies of stellar evolution it is common to
adopt X, Y and Z as indicative of the abundances by mass of hydrogen
(H), helium (He) and metals (Z), respectively.
The baryonic universe is madeup mainly of H and some He while only a
very small fraction resides in metals (all the elements heavier than
He), roughly 2%. However, the history of the growth of this small
fraction of metals is crucial for understanding how stars and galaxies
were formed and subsequently evolved; and last but not least, because
human beings exist only because of this small amount of metals!
We will focus then our attention is studying how the metals were formed and
evolved in galaxies, with particular attention to our own Galaxy.

The initial conditions for a model of galactic chemical evolution
consist in establishing whether: a) the chemical composition of the
initial gas is primordial or pre-enriched by a pre-galactic stellar
generation; b) the studied system is a closed box or an open system
(infall and/or outflow).

where k = 1-2 with a preference for k = 1.4 ± 0.15, as
suggested by
Kennicutt (1998a)
for spiral disks (see Figure 1), and
is a
parameter describing the star formation
efficiency, in other words, the SFR per unit mass of gas, and it has the
dimensions of the inverse of a time. Other physical quantities such as
gas temperature, viscosity and magnetic field are usually ignored.

Figure 1. The SFR as measured by
Kennicutt (1998a)
in star forming galaxies. The continuous line represents the
best fit to the data and it can be achieved either with the SF law in
eq. (6) with k = 1.4 or with the SF law in eq. (9). The short,
diagonal line shows the effect of changing the scaling radius by a
factor of 2. Figure from
Kennicutt
(1998a).

Other common parametrizations of the SFR include a dependence on the total
surface mass density besides the surface gas density:

(7)

as suggested by observational results of
Dopita & Ryder
(1994)
and taking into account the influence of the potential well in the star
formation process (i.e. feedback between SN energy input and star
formation, see also
Talbot & Arnett
1975).
Other suggestions concern the star formation induced by spiral density waves
(Wyse & Silk
1989)
with expressions like:

(8)

or

(9)

with gas
being the angular rotation speed of gas
(Kennicutt 1998a).
Also this law provides a good fit to the data of
Figure 1.

generally defined in a mass range of 0.1-100
M, where
a is the normalization constant derived by imposing that
0.1100m(m)
dm = 1.

The Scalo and Kroupa IMFs were derived from stellar counts in the solar
vicinity and suggest a three-slope function. Unfortunately, the same
analysis cannot be done in other galaxies and we cannot test if the IMF
is the same everywhere.
Kroupa (2001)
suggested that the IMF in stellar clusters is a universal one, very
similar to the Salpeter IMF for stars with masses larger than 0.5
M. In
particular, this universal IMF is:

(11)

However,
Weidner & Kroupa
(2005)
suggested that the IMF integrated over galaxies, which controls the
distribution of stellar remnants, the number of SNe and the chemical
enrichment of a galaxy is generally different from the IMF in stellar
clusters. This galaxial IMF is given by the integral of the stellar IMF
over the embedded star cluster mass function which varies from galaxy to
galaxy. Therefore, we should expect that the chemical enrichment
histories of different galaxies cannot be reproduced by an unique
invariant Salpeter-like IMF. In any case, this galaxial IMF is always
steeper than the universal IMF in the range of massive stars.

We define the current mass distribution of local Main Sequence (MS)
stars as the present day mass function (PDMF), n(m). Let
us suppose that we know n(m) from observations.
Then, the quantity n(m) can be expressed as follows: for
stars with initial masses in the range 0.1-1.0
M which have
lifetimes larger than a Hubble time we can write:

(12)

where tG ~ 14 Gyr (the age of the Universe). The IMF,
(m),
can be taken out of the integral if assumed to be
constant in time, and the PDMF becomes:

(13)

where <>
is the average SFR in the past.

For stars with lifetimes negligible relative to the age of the Universe,
namely for all the stars with m > 2
M, we
can write:

(14)

where m is the
lifetime of a star of mass m.
Again, if we assume that the IMF is constant in time we can write:

(15)

having assumed that the SFR did not change during the time interval between
(tG -
m) and
tG. The
quantity (tG) is the SFR at the present time.

We cannot derive the IMF betwen 1 and 2
M
because none of the previous semplifying hypotheses can be applied.
Therefore, the IMF in this mass range will depend on a quantity,
b(tG):

in order to fit the two branches of the IMF in the solar vicinity.
In Figure 2 we show the differences between a
single-slope IMF and multi-slope IMFs, which are preferred according to
the last studies.

The stellar yields, namely the
amount of newly formed and pre-existing elements ejected by stars of
all masses at their death, represent a fundamental ingredient to compute
galactic chemical evolution. They can be calculated by knowing stellar
evolution and nucleosynthesis.

I recall here the various stellar mass ranges and their nucleosynthesis
products. In particular:

Brown dwarfs: are stars with masses M
< 0.1
M which
never ignite H. They do not enrich the interstellar medium (ISM) in
chemical elements but only lock up gas.

All the elements with mass number A from 12 to 60 have
been formed in stars during the quiescent burnings.
Stars transform H into He and then He into heaviers until the
Fe-peak elements, where the binding energy per nucleon reaches a maximum
and the nuclear fusion reactions stop.
H is transformed into He through the proton-proton chain or the
CNO-cycle, then 4He is transformed into
12C through the
triple- reaction.

Elements heavier than 12C are then produced by synthesis
of -particles: they are
called -elements
(O, Ne, Mg, Si and others).

The last main burning in stars is the 28Si -burning
which produces
56Ni, which then decays into 56Co
and 56Fe.
Si-burning can be quiescent or explosive (depending on the temperature).

Explosive nucleosynthesis occurring during SN explosions
mainly produces Fe-peak elements. Elements
originating from s- and r-processes (with A > 60 up to Th and U)
are formed by means of slow or rapid (relative to the
- decay)
neutron capture by Fe seed nuclei;
s-processing occurs during quiescent He-burning,
whereas r-processing occurs during SN explosions.

In Figures 4, 5,
6, 7 and
8 we show a comparison between stellar yields
for massive stars
computed for different initial stellar metallicities and with different
assumptions concerning the mass loss. In particular, some yields are
obtained by assuming mass loss by stellar winds with a strong dependence
on metallicity (e.g.
Maeder, 1992),
whereas others (e.g. WW95) are computed by means of conservative models
without mass loss.
One important difference arises for oxygen in massive stars for solar
metallicity and mass loss: in this case, the O yield is strongly
depressed as a consequence of mass loss. In fact, the stars with masses
> 25 M
and solar metallicity lose a large amount of matter rich of He and C,
thus subctracting these elements to further processing which would lead
to O and heavier elements. So the net effect of mass loss is to increase
the production of He and C and to depress that of oxygen (see
Figure 9). More recently,
Meynet & Mader
(2002,
2003,
2005)
have computed a grid of models for stars with M > 20
M
including rotation and metallicity dependent mass loss. The effect of
metallicity dependent mass loss in decreasing the O production in
massive stars was confirmed, although they employed significantly lower
mass loss rates compared with
Maeder (1992).
With these models they were able to reproduce the frequency of WR stars
and the observed WN/WC ratio, as was the case for the previous Maeder
results. Therefore, it appears that the earlier mass loss rates made-up
for the omission of rotation in the stellar models.
On the other hand, the dependence upon metallicities of the yields
computed with conservative stellar models, such as those of WW95, is not
very strong except perhaps for the yields computed with zero intial
stellar metallicity (Pop III stars).

Figure 4. The yields of oxygen for massive
stars as computed by several authors, as indicated in the Figure. None
of these calculations takes into account mass loss by stellar
wind.

Figure 5. The same as Fig. 4 for
magnesium.

Figure 6. The same as Fig. 4 for Fe.

In Figures 7 and 8 we
show the most recent results of
Nomoto et al. (2006)
for conservative stellar models of massive stars at different
metallicities. While the O yields are not much dependent upon the
initial stellar metallicity, as in WW95 , the Fe yields seem to change
dramatically with the stellar metallicity.

Figure 7. The O yields as calculated by
Nomoto et al. (2006)
for different metallicities. These calculations do
not take into account mass loss by stellar wind.

Figure 8. The same as Figure 7 for Fe.

Figure 9. The effect of metallicity
dependent mass loss on the oxygen yield. The comparison is between the
conservative yields of WW95 for Z = 0.001 and Z = 0.02 and the yields
with mass loss of
Maeder (1992)
for the same metallicity. As one can see the effect of mass loss for a
solar metallicity is a quite important one.

There is a general consensus about the fact that SNeIa originate from
C-deflagration in C-O white dwarfs (WD) in binary systems, but several
evolutionary paths can lead to such an event. The C-deflagration
produces ~ 0.6-0.7
M of Fe
plus traces of other elements from C to Si, as observed in the spectra
of Type Ia SNe.

Two main evolutionary scenarios for the progenitors of Type Ia SNe have
been proposed:

Single Degenerate (SD) scenario (see Figure 10):
the classical scenario of
Whelan and Iben
(1973),
recently revised by
Han &
Podsiadlowsky (2004),
namely C-deflagration in
a C-O WD reaching the Chandrasekhar mass MCh ~ 1.44
M after
accreting material from a red giant
companion. One of the limitations of this scenario is that the accretion
rate should be defined in a quite narrow range of values. To avoid this
problem,
Kobayashi et
al. (1998)
had proposed a similar scenario, based on the model of
Hachisu et al. (1996),
where the companion can be either a red giant or a main sequence star,
including
a metallicity effect which suggests that no Type Ia systems can form
for [Fe/H] < -1.0 dex. This is due to the development of a strong
radiative wind from the C-O WD which stabilizes the accretion from the
companion, allowing for larger mass accretion rates than the previous
scenario. The clock to the explosion is given by the lifetime of the
secondary star in the binary system, where the WD is the primary (the
originally more massive one). Therefore, the largest mass for a
secondary is 8
M, which
is the maximum mass for the formation of a C-O WD. As a consequence, the
minimum timescale for the occurrence of Type Ia SNe is ~ 30 Myr
(i.e. the lifetime of a
8 M)
after the beginning of star formation. Recent observations in
radio-galaxies by Mannucci et al.
(2005;
2006)
seem to confirm the existence of such prompt Type Ia SNe.

The minimum mass for the secondary is 0.8
M, which
is the star with lifetime equal to the age of the universe. Stars with
masses below this limit are obviously not considered.
In summary, the mass range for both primary and secondary stars is, in
principle, between 0.8 and
8M,
although two stars of 0.8
M are
too small to give rise to a WD with a Chandrasekhar mass, and therefore
the mass of the primary star should be assumed to be high enough to
ensure that, even after accretion from a
0.8 M
star secondary, it will reach the Chandrasekhar mass.

Double Degenerate (DD) scenario:
the merging of two C-O white dwarfs, due to loss of angular momentum caused
by gravitational wave radiation,
which explode by C-deflagration when MCh is reached
(Iben and Tutukov
1984).
In this scenario, the two C-O WDs should be of ~ 0.7
M in
order to give rise to a Chandrasekhar mass after they merge, therefore
their progenitors should be in the range (5-8)
M. The
clock to the explosion here is given by the lifetime of the secondary
star plus the gravitational time delay which depends on the original
separation of the two WDs. The minimum timescale for the appearance of
the first Type Ia SNe in this scenario is a few million years more than
in the SD scenario (e.g. ~ 40 Myr in
Tornambé &
Matteucci 1986).
At the same time, the maximum gravitational time delay can be as long as
more than a Hubble time. For more recent results on the DD scenario see
Greggio (2005).

Figure 10. The progenitor of a Type Ia SN
in the context of the single-degenerate model (Illustration credit:
NASA, ESA, and A. Field (STSci)).

Within any scenario the explosion can occur either when the C-O WD
reaches the Chandrasekhar mass and carbon deflagrates at the center or
when a massive enough helium layer is accumulated on top of the C-O
WD. In this last case there is He-detonation which induces an off-center
carbon deflagration before the Chandrasekhar mass is reached
(sub-chandra exploders, e.g.
Woosley & Weaver
1994).

While the chandra-exploders are supposed to produce the same
nucleosynthesis (C-deflagration of a Chandrasekhar mass), they
predict a different evolution of the Type Ia SN rate and different
typical timescales for the SNe Ia enrichment. A way of defining the
typical Type Ia SN timescale is to assume it as the
time when the maximum in the Type Ia SN rate is reached
(Matteucci &
Recchi, 2001).
This timescale varies according to the chosen progenitor model and to
the assumed star formation history, which varies from galaxy to galaxy.
For the solar vicinity, this timescale is at least 1 Gyr,
if the SD scenario is assumed, whereas for elliptical galaxies, where
the stars formed much more quickly, this timescale is only 0.5 Gyr
(Matteucci &
Greggio, 1986;
Matteucci &
Recchi 2001).